Accurate wavelength lasers are needed as transmitter sources for Wavelength Division Multiplexed (WDM), Dense Wavelength Division Multiplexed (DWDM) fiber-optic communications, pump lasers for various media such as Erbium doped optical fiber amplifiers (EDFA) or solid state lasers, illumination sources for differential spectroscopy, and other applications requiring compact, precise wavelength sources. In telecommunications, semiconductor lasers have been used because of their small size, low cost, high efficiency, and ability to be modulated at high speed. These sources typically operate in the 1.3 .mu.m band, which is at the zero dispersion point of conventional optical fibers, and more recently in the 1.55 .mu.m band because of the loss minima and the availability of EDFA's in this wavelength band.
Dense wavelength division multiplexed optical networks increase the information carrying capacity of a transmission system by loading multiple channels of differing optical frequencies onto a single optical fiber. The channel density of commercial DWDM systems has increased dramatically resulting in narrower frequency spacing between channels. This close channel spacing can be sensitive to crosstalk caused by frequency drifts in which a channel interferes with an adjacent channel. These drifts may be caused by phenomena similar to those occurring due to short-term drift and long-term aging.
While narrow frequency spacing between channels is desirable, prior art methods of achieving narrowed frequency spacing require that the thickness of an etalon optical filter be increased. This is related to the physics of a Fabry-Perot (FP) cavity. For example, in order to achieve 50 GHz spacing between channels using prior art methods, a 2-mm thick etalon is required; however, due to design constraints within a laser module, a 1-mm thick etalon is desired. Using prior art techniques, the minimum achievable spacing between channels using a 1-mm thick etalon is 100 GHz.
The free spectral range (FSR) of the FP etalon is determined by measuring the distance in optical frequency between a pair of adjacent peaks in the transmission spectrum. The transmission occurs at frequencies spaced c/2d apart, where c is the velocity of light and d is the distance between the reflective surfaces of the etalon. The output of a laser has a wavelength, and the point at which that wavelength and the transmission peak of the etalon cross a reference point is normally called a grid channel. In prior art systems this grid is defined at the peaks of the etalon function. The grid of channels is presently defined by the International Telecommunications Union (ITU) at 100 GHz channel spacing with 50 and 25 GHz spacing possible in the near future. Channel spacing will decrease in the future to allow more wavelengths to fit within the fixed bandwidth of the EDFA. Laser temperature determines which grid channel region a laser wavelength will be in at any given point in time.
Systems for stabilizing optical frequencies are employed within DWDM optical networks. Typically, these systems detect an optical frequency using a frequency discriminator in closed loop feedback with an optical source. Optical frequency information is translated into an error signal that is used to correct the source frequency to within some system-specific tolerance. It is well known that a FP etalon exhibits periodic optical transmission characteristics. It is also known that frequency discriminators with characteristics which are precisely aligned to the channel frequency of a DWDM system can be used to advantage, such as for frequency filtering, within such systems. Finally, it is known that FP etalons, used as discriminators, can be employed within DWDM systems when the FSR of the etalon is equal to the channel separation and the transmission peaks of the etalon are aligned with channel frequencies of the system. In prior art systems the laser channels will fall at intervals which are equal to the period of the etalon filter if the FSR, etalon angle to the laser source, and etalon temperature are properly matched to the absolute channel grid.
In addition to stabilizing an optical frequency on a particular grid channel, it may also be desired to switch from one grid channel to a different grid channel. Prior art systems allow changing between channels by simply temperature tuning the laser diode. The problem with these methods is that the laser has a temperature dependence of approximately +0.09 nm/.degree. C. Precise 100 GHz wavelength control based strictly on temperature tuning is acceptable for the best of lasers, such as the type E2500 Electroabsorption Modulated Isolated Laser Module (EM-ILM) produced by Lucent Technologies, Inc., but is expected to be insufficient for a 25-50 GHz spaced system over an expected 25 year system life because of the small margin for drift in the lasers and filters used in these narrowly spaced systems.